Internal Flash not working-Canon Eos 40D

I took pictures at a birthday party with an external flash. No problems. Then we went to a girl scout event and I took pictures with the internal flash. No problems. I come home and want to take a picture and on auto, or any automatic flash setting, the flash is not coming up. I hit the fash pop up button: nothing happens. There is no code, and it isn't saying the flash is blocked. It is just shooting at a very slow speed to compensate for the low light because the flash isn't coming up. There was no trama to the camera to cause this. Thank you for your help. I feel very upset that it isn't working and I can't figure why. Ruth